This invention relates to fuel cells utilizing a liquid electrolyte contained in a porous matrix between the electrodes of each fuel cell and, more particularly, to apparatus for supplying liquid electrolyte to the fuel cell stacks in a fuel cell module for replenishment purposes.
Among the various types of fuel cell systems are those which include subassemblies of bipolar plates between which are supported electrodes and a liquid electrolyte contained in a porous matrix. The subassemblies, herein referred to as fuel cells, are oriented one atop another and electrically connected in series, or otherwise, to form a fuel cell stack. During exemplary operation of the fuel cell, the exothermic reaction of hydrogen and oxygen produces heat, electrical energy, and water, making cooling of the cell components necessary in order to maintain component integrity. Liquid or gaseous cooling fluids have therefore been passed through the stack to remove heat. Accordingly, three fluid mediums, a fuel, an oxidant, and a cooling fluid, flow in some manner into and out of the fuel cell stack.
Optimum performance of a fuel cell results when the porous matrix is wetted by the liquid electrolyte and the remaining area around the matrix is wetted to maintain the matrix in the electrolyte-wetted condition. Wetting of the matrix is required to transport ions and to provide a seal which prevents mixing of the reacting gas streams. After a period of operation, the fuel cells suffer performance degradation due to gradual loss of the electrolyte by evaporation to the gas streams, causing a drying-out at the electrolyte matrix-electrode interfaces.
Electrolyte replenishment is currently carried out during periods of shutdown since the wicking characteristics of the matrices and flow resistance in the acid passages make replenishment time consuming. If electrolyte is added faster than it can be accommodated by the fuel cell, a large hydrostatic head is produced and the excess electrolyte is forced into the electrodes of the cell with resultant flooding of the electrodes, adversely affecting the electrochemical performance of the cell.
Commercial fuel cell applications require extended periods of continuous operations resulting in the elimination of the shutdown periods during which make-up electrolyte has been added. A system for replenishing electrolyte during fuel cell operation is desirable.